PHOTOSYNTHESIS

Computer-enhanced photos ofnature hard at work

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THE WORLD'S LEADING SCIENCE WRITER ASKS US TO REFLECT WITH HIM ON HOW SIMPLE
MATERIALS SUCH AS THE DROPLETS OF WATER ABOVE COMBINE WITH OTHER ELEMENTS IN A
PROCESS THAT GIVES US THE FOOD WE EAT AND THE AIR WE BREATHE.

Think for a moment of the things you don't think about.

You are breathing and, with every breath, you consume oxygen and produce
carbon dioxide. The process is absolutely essential to life, and you
have been doing this during all your existence. So has every other person
on Earth. In fact, all animal life, from whales to water fleas and from bulls
to beetles, has been doing this very same thing and has been doing it
for at least 2 billion years.

The question is, How is it that we've never used up all the oxygen and died
for lack of it?

Again, it is essential for life that we eat, and what we eat is food (never
mind the chemistry). In order to get the energy we need to maintain
life, we combine the food with oxygen to produce water and carbon
dioxide. We all do it constantly. Why haven't we run out of food as well
as oxygen?

Obviously, the only answer is that something exists that reverses the
process. All animal life is marked by the following: Food plus oxygen
forms water plus carbon dioxide.

Somewhere on Earth, therefore, there must be a reverse process that can be
described as water plus carbon dioxide forms food plus oxygen.

And there is! Green plants, exposed to sunlight, convert water and carbon
dioxide into food and oxygen, and the balance has thus been preserved
through all the existence of life on our planet.

Since the conversion of food and oxygen to carbon dioxide and water
produces the energy that maintains animal life, the reverse process,
conducted by green plants, must consume energy. (You get nothing for
nothing in this world.) Where does the energy come from?

It comes from the energy of visible light. As an energy source, light is
perfect for two reasons: First, it is unimaginably copious. The earth is
constantly bathed in sunlight. Second, visible light is a mild, unintense
form of energy that does little or no harm.

About 2 billion years ago, tiny bacterialike cells called cyanobacteria
(which still exist on Earth) developed the chemistry to handle sunlight
and make use of its energy to form food and oxygen. The ocean slowly
filled with food, and the air slowly filled with oxygen, and eventually the
groundwork was laid for the development of life more complex than bacteria.

Some of the complex life forms that eventually formed were green
plants, and some were animals. Between the two of them the balance was
maintained.

What is it in green plants that does the trick? It is a compound called
chlorophyll, which rather resembles the heme in animal hemoglobin.
Chlorophyll is a bit more complicated and has a magnesium atom where
heme has an iron atom. But it is not simply chlorophyll that does the
trick. If it were, we could isolate chlorophyll and put it to work.
Instead the chlorophyll exists in intricate and complex little organelles
within the cell, organelles called chloroplasts, and it is these that
perform the process called photosynthesis ("put together by light").

Using modern techniques-powerful microscopes, radioactive tracers, and so
on-chemists have managed in the last generation to work out many of the
detailed changes that take place within the chloroplasts. They have
learned how certain chemicals change into others and how units of
sunlight are incorporated into the process so as to supply the necessary
energy.

However, we have not learned how to repeat this process with anything
less complex than a chloroplast, so that not all the knowledge we have
yet gained will suffice to have us replace, or supplement, the work of
the green plants.

This is not to suppose that someday we won't, that the day might not
come when we can set up artificial systems for the conversion of
water and carbon dioxide into food and water. If so, it is precisely such
artificial systems that may make long human flights through space
possible and may enable us, someday, to reach the stars.